Ultrasonic and laser welding offer opportunity for joining plastic

Paul Boughton

Ultrasonic welding and laser welding offer new opportunities for joining plastics. Product manufacturers are under pressure to reduce the cost of assembling plastic components but without compromising quality. For those wishing to avoid adhesives, there are some new opportunities worth investigating in the fields of ultrasonic welding and laser welding, as Jon Severn reports.

Plastic components are found in most mass-produced goods today, but the choice of fastening method will depend on the component's function, the application and whether the component needs to be removed for maintenance or recycling at the end of the product's life. However, a major concern for designers is how the assembly can be manufactured most economically.

For those products where a permanent joint is required, and where adhesives are unsuitable, designers are generally left to choose between several different types of welding.

For the purposes of this present article, we will consider some of the latest developments in ultrasonic welding and laser welding, though it is appreciated that other processes - such as non-contact hot plate welding and infra red lamp welding - may be more appropriate for some applications.

Ultrasonic welding has become very popular for a wide range of applications, from medical devices to automotive components, consumer products and toys.

The process is clean, fast, results in little or no flash, and can be readily automated for high-volume production.

A typical ultrasonic welding system will consist of a nest in which the part is supported, a power supply, converter (or transducer - which converts the electrical energy into ultrasonic vibrations), booster (an amplitude modifying device) and a horn (or sonotrode - the tool that contacts the workpiece).

In addition, there will be a means of applying a clamping force to the joint and, of course, a controller.

Hermetically sealed joints

As well as being suitable for welding two components of the same material, with hermetically sealed joints if required, the ultrasonic process can be used for spot welding or to form heads on stakes so that components of different materials can be locked in place mechanically. Similarly, ultrasonic technology can be used on metal components such as threaded inserts to create strong, relatively low-stress joints in thermoplastic components.

There have always been limitations to what can be achieved with ultrasonics welding - such as a maximum joint length of around 250mm - and one type of application that has proved difficult is the welding of membranes to other components; here, the membrane can behave as a diaphragm (drum skin) and vibrate along its axis, thereby incurring damage.

In the medical industry in particular, where there is concern about loose fragments that might be inhaled in certain applications, such damage is unacceptable. Alternative methods have therefore had to be used for joining, say, filter media to housings for medical devices. However, Telsonic has developed an innovative solution to this problem that enables high-quality joints to be made without damaging the membrane component.

Welding with a twist

Traditional ultrasonic welding has the main system components aligned longitudinally. But with the patented Telsonic Soniqtwist technology, the converter is mounted tangentially to the booster, so the sonotrode exerts torsional vibrations on the workpiece rather than axial vibrations (Fig. 1). Membranes are therefore not subjected to any axial vibration, so they can be welded without risk of damage.

In addition, some electronic components can be welded using the Soniqtwist technology, whereas conventional longitudinal ultrasonic welding can cause internal damage (Fig. 2).

According to Telsonic, the Soniqtwist technology can also be used to join non-ferrous metals to ceramics or glass, with the ultrasonic energy being converted into a molecular surface bond between the dissimilar materials.

Another development in ultrasonics from Telsonic is the IPA 3505 (Integrated Power Actuator) for spot welding or multi-point welding (Fig. 3).

Traditional ultrasonic welding systems require a separate generator for each welding head or, alternatively, fewer welding generators and a means of switching so that each generator can serve multiple welding heads - albeit with penalties in terms of increased system complexity and longer cycle times.

In contrast, the PA 3505 is a compact, lightweight unit that has a fully integrated actuator, miniature generator and converter. Telsonic says that this unit is cost-effective for use as a standalone fastening device or in larger numbers in fully automated welding systems.

The IPA operates at an industry-standard frequency of 35kHz with a power and force capability of 500W and 350N, respectively. Each self-contained unit has an adjustable 50mm stroke with integral damping and upper and lower stroke sensors.

Laser welding

Target markets for the IPA are the automotive industry, together with industrial and packaging sectors. In the automotive sector, for example, Telsonic says the IPA is useful for oblique face spot welding in heat-sensitive areas such as attaching insulation and sound-proofing materials, plus it offers savings in space and cost for pin swaging and penetrative spot welding of interior trim parts such as A, B, C and D pillars, carpet areas, glove box assemblies and load space components, together with exterior components such as wheel arch liners and engine bay covers.

Already IPA units have been used in the manufacture of door trim panels, where as many as 44 units have been incorporated within a compact machine that produces left-hand and right-hand panels simultaneously.

Furthermore, the compact dimensions and low weight (4.2kg) mean the IPA can be readily mounted on a robot arm for spot welding operations on complex three-dimensional components.

Turning to laser welding, there are generally two types that designers could use: direct laser welding and transmission laser welding. Direct laser welding would normally be restricted to the welding of thin films, as the radiation from the CO2 laser is readily absorbed by the plastic.

In transmission laser welding, Nd:YAG, fibre and diode lasers are used instead, and the radiation from these is less readily absorbed by plastics; by directing the laser at the joint through a transmissive plastic and by selecting a second plastic material that will absorb the laser energy, heat can be generated at the joint interface.

However, if the designer cannot specify an absorbing plastic for the second component, then an alternative is to apply an opaque surface coating at the joint.

Transmission laser welding is capable of welding thicker parts than direct welding, without marking the outer surfaces of the components.

Transmissive plastics

Where two transmissive plastics are to be joined, the traditional approach is to use a carbon black absorber, but today there is a clear product called Clearweld that absorbs infrared.

Invented and patented by TWI, and subsequently licensed to and commercialised by Gentex Corporation, Clearweld enables virtually invisible welds to be made in clear or coloured plastics.

Unless the depth of field is sufficient to tolerate variations in the distance between the laser and the workpiece, positioning and flatness are crucial for a good result.

Most laser welding applications are therefore on components with a joint line that is in one plane. However, at the 2010 Lasys trade fair, LPKF exhibited its new Twinweld3D hybrid welding machine that is said to offer high-quality welds and fast processing times. Based around a robot, the Twinweld3D hybrid welding system enables designers to create freeform three-dimensional car headlights and taillights that LPKF says would be impossible to weld using conventional methods (Fig. 4). Further savings in time and cost are possible because tempering is not usually required after welding with this system.

LPKF believes there is a market trend towards the use of large, freeform three-dimensional mouldings, so it has developed the Twinweld3D to meet this need. Another trend, according to LPKF, is for a greater use of welding to join plastic micro-components, hence the company is also active in this area.

Versatile process

Laser welding is no longer the 'space age' technology that it once was, and it is certainly becoming a much more versatile process. At the same time, ultrasonic welding can now be used for a wider variety of applications than those for which it was suitable in the past.

Design engineers looking to reduce manufacturing costs without compromising quality would do well to look at both of these joining technologies when considering both new projects and ways to remove cost from existing products.

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